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1 172 The " Collar Bone" in the Mammalia. [December, 

THE SIGNIFICANCE OF THE "COLLAR BONE" IN 
THE MAMMALIA. 

BY SPENCER TROTTER, M.D. 

WHEN running through a series of forms in animal life a 
structure is found fully developed in some and in others 
rudimentary or entirely wanting, we are led to consider the 
causes acting upon the structure through the life of the animal 
which has brought about the condition of development or non- 
development present. Every fully developed tissue in an organ- 
ism is needed or it would not be there; and just so soon as by 
increasing change in life and habits, it becomes a factor of less 
and less importance to the animal ; it fails more and more to 
attain its former standard of development, and in time falls back 
to the primitive condition from which it arose and finally dis- 
appears. 

The "collar bone," or clavicle is an unstable factor in the 
shoulder girdle ; by this I mean an element not always found 
present throughout vertebrates, and its presence, absence or rudi- 
mentary condition, is in relation to the life of the animal. 

In this article it is my intention to note, principally among the 
mammals, the relation which the clavicle bears to the various 
modes of life, but before taking up the consideration of individ- 
ual forms, reviewing briefly the general anatomy of the part. 

The shoulder girdle consists first of the scapula, or " shoulder 
blade," a more or less irregular plate of bone preformed in car- 
tilage and ossifying from two centers, the dorsal or scapular, the 
ventral or coracoid, in position against the anterior thoracic ribs, 
its long axis varying in inclination. 

Its ventral end terminates in the " glenoid cavity," a ridge and 
process, spinous and acromion, on its outer surface are more or 
less developed in different forms, and in all mammals above 
the Ornithodelphia the coracoid is reduced to a mere process. 

Second, the clavicle, when present, preformed in fibrous tissue 
extends as a bar of bone from the acromion process above the 
glenoid cavity to the manubrium sterni, forming a strong support 
to the girdle and an extended surface for ligamentous and mus- 
cular attachments. Mechanically considered the shoulder girdle 
(by using the term " girdle " both sides of the body are implied) 
is nothing more than the fulcrum of which the fore limb is the 
lever of the third kind, and its specialization is in direct relation 



1885.] The " Collar Bone" in the Mammalia. 1173 

to the amount and character of the work done. The clavicle 
first appears in the ganoid fishes as a secondary apparatus devel- 
oped in connection with the primary cartilaginous scapular arch. 
In the teleost or bony fishes it becomes a more important element 
in the shoulder girdle, having connection with the skull and the 
opposite sides joining in the ventral median line. 

It undergoes a reduction and becomes of less significance in 
Amphibia and Reptilia, but in birds it assumes an important 
position in relation to flight, the entire shoulder girdle in fact 
being specialized to meet the conditions incident to aerial loco- 
motion. The coracoids — large, strong bones — act as braces ; the 
clavicles, peculiarly modified, are united at their sternal ends into 
one bone, the furculum, or " merry thought," which, as Owen has 
pointed out, acts as an elastic, bony arch opposing the force inci- 
dent upon the downward stroke of the wing in flight, thus aiding 
the humeri, or arm bones, to regain their former position in the 
succeeding upward or counter stroke with as little loss of time 
and energy as possible. In terrestrial forms (grouse, fowls, etc.), 
where flight is sustained for short intervals only, the arch is nar- 
rower and the structure more slender and delicate. 

Taking up the Mammalia with a view to ascertaining the rela- 
tionship existing between the development of the clavicles and 
the work done by the fore limbs, let us start with the following 
general proposition as a basis for our observations, namely, that 
those animals which have the fore limbs specialized over the hind 
limbs in relation to work, possess a clavicle, and where the hind 
limbs are the most highly specialized, the clavicles are rudiment- 
ary or entirely wanting. 

Leaving the ornithodelph mammals (the Australian duck-bill, 
Echidna, etc.), with their bird-like shoulder girdle, we note the 
presence of more or less well developed clavicles in all the mar- 
supials, with the single exception of the "bandicoots" (Pera- 
melidae), where it is wanting, and its absence may be accounted 
for from the fact that the bandicoots from their terrestrial mode 
of life, nesting in hollow places and feeding on insects, roots 
etc., have for a long time had their fore limbs subjected to less 
complex conditions than the arboreal and predatory families, the 
opossums (Didelphidse), the Uasyuridse, the phalangers (Phalang- 
istidae) and the kangaroos (Macropodidae), which constitute the 
rest of the order. In the kangaroos the clavicles, though pres- 



1 1 74 The "Collar Bone" in the Mammalia. [December, 

ent, are slender and delicate, the weak fore limbs coming into use 
in manipulating the "pouch," etc., thus throwing a variety of 
motion into the shoulder joint, which explains the presence of 
clavicles in an animal whose locomotion is almost entirely per- 
formed by the hind limbs. 

Among the Insectivora, a large order of wide geographical 
range and rather uniform life, the majority of species being ter- 
restrial and fossorial or burrowing, we have the clavicles well 
developed with but one exception, that of Potamogale velox, a rare 
form from Western Africa, and this is the only insectivore which 
is almost entirely aquatic. It measures about two feet in length 
with a long cylindrical body, tapering continuously into a thin, 
laterally compressed tail, which is the main propulsive power 
when swimming, the short legs with their unwebbed feet drifting 
back against the body. Another form, Myogale, from the streams 
of Southeastern Russia, is natatorial and possesses a clavicle, but 
the feet are all webbed and come into play along with the tail as 
organs of propulsion. 

The moles, Talpidae, are eminently fossorial, their excavations 
being everywhere known ; an East Indian squirrel- like form is 
arboreal ; all the rest are terrestrial and fossorial. 

The order Edentata is divided into two primary groups, the 
" leaf-eaters " (Phytophaga) and the " insect-eaters " (Entomoph- 
aga). The Phytophaga comprise the " sloths," curious, arboreal 
forms inhabiting the South American forests, represented by only 
two living genera ; they are entirely arboreal, making character- 
istic progression among the tree-tops on the leaves of which they 
feed. In the two-toed sloth, or " unau " (Cholcepus), the clavi- 
cles are well developed ; in the " ai," or three-toed variety (Brady- 
pus), they are small, rudimentary, having lost their sternal attach- 
ments and evidently undergoing a retrogressive change, due 
probably to some variation in the animal's life and habits which 
brings the part into less active use. 

In the Entomophaga (ant-eaters, armadillos, etc.) we find the 
clavicles fully developed in the climbing two-toed ant-eater Cy- 
clothurus, a small South American species ; also in the Cape ant- 
eater, or "aard-vark," Orycteropus, a burrowing form from the 
Cape of Good Hope; while in the great ant-eater, Myrme- 
cophaga, a strictly terrestrial form but not fossorial, the clavicles 
are absent, the long claws tearing open the ant hills and termites' 



1885.] The "Collar Bone " in the Mammalia. 1 175 

mounds, and the narrow, flexible tongue securing the food, stand 
in place of the burrowing habit, thus bringing the shoulder joint 
under much less active conditions. In the South American 
armadillos, which are more omnivorous and are " diggers," the 
clavicles are developed. The "pangolins," scaly covered ani- 
mals, mainly terrestrial and rolling themselves in a ball on the 
approach of danger, have no clavicles. 

In the order Rodentia we find all the Sciuromorph group with 
fully developed clavicles ; the squirrels are either arboreal or bur- 
rowers; the marmots, Arctomyinae, are typical burrowers, while 
the beaver, though eminently aquatic, brings the fore limbs into 
active use in the construction of its dams and lodges. 

The Myomorpha (rats, mice, gophers, etc., typical gnawers 
and diggers) have well developed clavicles, the gnawing habit 
bringing the fore limbs and shoulder girdle actively into use to 
steady the head and neck and the object which is being worked 
upon. 

In the Hystricomorpha (porcupines, cavies, etc.) clavicles are 
found developed in the climbing tree-porcupines (Sphingurinae), of 
which our Canadian species (Erethizon) is a member, and absent 
in the large African porcupine and all others of the Hystricine 
group, which is strictly terrestrial but not fossorial. 

The chinchillas have well developed clavicles, the horse-trip- 
ping burrows of the " Pampas Viscacha " being too well known 
to the riders of that region. The agoutis (Dasyproctidae), inhab- 
iting the banks of streams in the South American forests, semi- 
aquatic and sub-ungulate (i. e., having the feet partially encased 
in a horny sheath or hoof) have rudimentary clavicles. The 
Caviidae (guinea pigs) and the " capybara," the largest living 
rodent, semi-aquatic, with webbed feet, have imperfect clavicles. 
, The Lagomorpha consists of two living families, the Lagomyi- 
dae, small burrowing animals called " picas," inhabiting Alpine 
regions, and the Leporidae (hares and rabbits) ; the former family 
having well developed, the latter rudimentary clavicles. The 
hares are typical runners, solitary, never burrowing, depending 
for safety in speed and making their characteristic " form " ; the 
species known as the rabbit, however, differs largely in habits 
from the other members of the family, being more or less gre- 
garious, living in " warrens " which it burrows out, and having 
much less capacity for speed. Clavicles are developed in both ; 

vol, xix.— No. xii. 77 



1 176 The "Collar Bone " in the Mammalia. [December, 

very rudimentary in the hares, and in the rabbits existing as a 
bone incomplete at the ends, a condition which we would expect 
to find from the variation in life habits. 

Taking up the large order of "hoofed " animals, the Ungulata, 
we note the complete absence of the clavicles in all the forms, 
not a vestige of the bony structure being found in any one of 
them. They are the " runners " par excellance, and if we may 
use the expression, live on their legs, the capacity for speed and 
endurance being one of the chief factors aimed at by natural 
selection in maintaining and perfecting the species. 

Here we find the fore limbs subserving the hind limbs in rela- 
tion to work ; the latter are the main motor power in running, 
while the fore limbs act chiefly as supporters and guiders ; hence 
the entire absence of clavicles. The marine Mammalia, with their 
rudimentary limbs, have no clavicles. 

In the flesh-eating animals, Carnivora, the clavicles are always 
rudimentary, and in many cases entirely wanting. They are bet- 
ter developed in the cats, Felidse, than in any other family of the 
order (though the articular extremities of the bone are lost), for 
the clutch which follows the spring in securing their prey, brings 
the fore limbs into more active use than in the dogs and wolves, 
Canidae, which run their quarry to " the death," and then pull it 
down by force of numbers. Many of the. cats, too, frequent 
trees, but they spring from place to place and do not properly 
climb. Several species of bears also climb, but the habit is 
more acquired than natural, the fore limbs clasp the trunk while 
the hind ones secure a foothold and, so to speak, shove the ani- 
mal upward, very different from the active movements of a nat- 
ural climber. 1 

In the bats, Chiroptera, as we would expect from their mode 
of life, the anterior limbs being the main factors in their flight, 
clavicles are developed, long, curved and strong. 

The arboreal Lemurs also have them developed. 

In the order Primates we have the clavicles present in all, from 
the arboreal monkeys up through the anthropoid apes to man, 
where the anterior limbs attain such perfection, with ball and 

1 The badger (Taxidea) is a striking exception to the general rule, being an emi- 
nently fossorial animal yet devoid of clavicles. The fore feet are armed with ex- 
ceedingly long, slightly curved claws, probably bringing greater leverage on the 
wrist and elbow joints and thus partially supplementing the absence of clavicles. 



1885.] Pear Blight and its Cause. 1 1 77 

socket shoulder joint capable of such varied and extensive motion, 
with a high degree of pronation and supination of the fore arm, 
and last, but not least, the wonderfully specialized hand with its 
thumb opposable to each of the four digits. 

After this brief survey, and with the forementioned proposition 
in view, viz., the correlation existing between the development of 
the clavicle and the work done by the fore limbs, we are left to 
draw the rational conclusion that the subject under consideration 
is one of use and disuse of parts, as Darwin has so clearly pointed 
out in his chapter on rudimentary organs in the Origin of Spe- 
cies. The facts we have noted in our hasty glance at the Mam- 
malia confirm this, in the more or less perfect development of 
clavicles in arboreal, fossorial, aerial and all other forms where 
the fore limbs are the active, aggressive pair in the life of the 
animal, and their absence or rudimentary condition in the hoofed 
animals, the marine species and all others where the anterior pair 
take a secondary place in the work done by the limbs. 

As there is, of course, no actual disuse of a part as a whole 
(the nearest approach to this being in marine forms), a simple, 
uncomplicated motion existing, with little strain at the shoulder 
joint, the parts require less support and fewer points for ligament- 
ous and muscular attachment than where the movements are 
more complicated and the strain more severe. Consequently we 
have a greater or less differentiation in the elements of the shoul- 
der girdle as the case may be, and the clavicle, holding as it does 
a position of secondary importance, is the unstable, variable ele- 
ment. 



-:o:- 



PEAR BLIGHT AND ITS CAUSE. 

BY J. C. ARTHUR. 

PEAR trees in this country are subject to an endemic disease 
that, owing to its malignancy and frequent occurrence, is well 
known to cultivators and fairly well discriminated by them. It is 
known both z.s pear blight and jire blight, and the same disease' in 
the apple and quince is also called twig blight. The term blight 
is applied to many kinds of plant diseases, and especially to those 
that eventually kill without rendering the cause conspicuous ; it 
is also the name of a class of disease-producing fungi. The pear 
malady bearing this name is, however, a specific disease, although